Auto CPAP system profile information

ABSTRACT

The present invention relates to systems and methods for automatically and continuously regulating the level of nasal pressure to an optimal value during OSA treatment. OSA therapy is implemented by a device which automatically re-evaluates an applied pressure and continually searches for a minimum pressure required to adequately distend a patient&#39;s pharyngeal airway. For example, this optimal level varies with body position and stage of sleep throughout the night. In addition, this level varies depending upon the patient&#39;s body weight and whether or not alcohol or sleeping medicine has been ingested.

RELATED APPLICATION

This application is a continuation, of application Ser. No. 08/842,981filed on Apr. 25, 1997, now abandoned which is a continuation of Ser.No. 08/093,131 filed on Jan. 29, 1993, now U.S. Pat. No. 5,645,053issued Jul. 8, 1997 which is a continuation of Ser. No. 07/868,199 filedon Apr. 14, 1992, now abandoned, which is a continuation of applicationSer. No. 07/791,733 filed Nov. 14, 1991, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to systems and methods fortreating sleep disorders. More particularly, the invention relates tosystems and methods for treating obstructive sleep apnea using anadaptive control system for applying air pressure to the nasal airway.

2. State of the Art

Obstructive sleep apnea (OSA) is a newly recognized disease thatvictimizes 15% of adult males. The disorder arises during sleep when thevictim undergoes repeated cessation of breathing. This cessation resultsfrom an obstruction of the throat air passage (pharynx) due to acollapse of the throat air passage. Repeated cessation of breathingreduces blood oxygen and disturbs sleep. Reduction in blood oxygen cancause heart attacks and strokes. Sleep disturbances can produceexcessive daytime sleepiness, a leading cause of auto accidents.

Medical research over the past decade has provided only one effectiveand practical approach to OSA therapy, known as nasal continuouspositive airway pressure (CPAP). In this therapeutic approach, apatient's nose is covered with a mask that forms a pressure seal withthe surrounding face. While the patient sleeps, the mask is pressurizedto a level that distends the collapsible throat air passage, therebypreventing obstruction.

This therapeutic approach provides two significant advantages: it isuniformly effective and it is entirely benign. A major disadvantage ofthis approach is that the patient must remain overnight in a hospitalsleep center to undergo a full night polysomnography study with thepressure mask in place to determine the therapeutic level of pressure. Afurther disadvantage of this approach is that the pressure delivered tothe patient during the polysomnography study is constant and fixed atthe prescribed level, even though the patient's requirements may varythroughout the night and from night-to-night.

The overnight study represents a major bottleneck to treating hundredsof thousands of patients with OSA because it typically requires two fullnight polysomnographic studies for each new patient: one to establishthe diagnosis (diagnostic-polysomnogram) and another to establish theaforementioned therapeutically optimal pressure(therapeutic-polysomnogram). The therapeutic polysmnographic study isnecessary to determine the minimum level of pressure required to producea patent pharyngeal airway (i.e., to determine the necessary therapeuticpressure required for properly treating the patient). These studies,performed in a specialized hospital sleep center, allow a specialist tospecify the pressure to be used when prescribing nasal CPAP therapy. Forthis reason, the therapy cannot be prescribed by an internist or generalpractitioner.

Due to the requirement of two night polysomnographic studies, hospitalsleep centers are crowded even though only a small percentage of OSAvictims are presently being treated. Further, the significant cost ofthe overnight polysomnographic study by a hospital sleep centerrepresents a significant obstacle to diagnosing and treating the largepopulation of sleep apneics. The backlog of undiagnosed and untreatedOSA patients thus represents a substantial public health problem.

To address the foregoing drawbacks of existing approaches to diagnosisand treatment of OSA, recent commercial technology provides overnight,unattended monitoring of breathing in the patient's home. Suchunattended monitoring generally permits the physician to diagnose OSAwithout requiring a diagnostic overnight study in the hospital sleepcenter. However, a hospital sleep center is still required forestablishing the therapeutically optimal pressure of nasal CPAP in eachpatient. Accordingly, medical practitioners have been slow to use thenew monitoring technology for diagnostic purposes since the patientmust, in any case, be referred to a sleep center for a full nighttherapeutic polysomnographic study.

Accordingly, it would be desirable to render the diagnosis and therapyof OSA more practical, convenient and inexpensive. To achieve this end,a method and system for automatically establishing the desired nasalCPAP pressure is needed. More particularly, a positive airway pressuresystem is required which will allow a physician, following diagnosiswith convenient monitoring technology, to prescribe nasal CPAP withoutspecifying the pressure.

SUMMARY OF INVENTION

The present invention is therefore directed at providing a practical,convenient and cost-effective system for diagnosing and treating OSA.Further, the invention is directed to portable systems and methods forautomatically and continuously regulating the level of nasal pressure toan optimal value during OSA treatment. OSA therapy is implemented byautomatically applying an appropriate pressure level to a patient. Theapplied pressure is continuously re-evaluated and optimized. Thisoptimal level varies with body position and stage of sleep throughoutthe night. In addition, the required pressure varies depending upon thepatient's body weight and whether or not any deleterious substances,such as alcohol or sleeping medicine, have been ingested.

Thus, the present invention relates to systems and methods foradaptively providing continuous positive airway pressure to an upperairway system by detecting airflow data in the upper airway system atpredetermined increments of time; averaging said airflow data over asecond period of time which includes a plurality of said predeterminedtime increments; determining non-respiratory airflow using said averageddata; identifying periods of inspiration and expiration using saidnon-respiratory airflow data; extracting information or features fromsaid airflow data; and continuously adjusting pressure in said upperairway system.

In a preferred embodiment, a portable adaptive control system isprovided which continually searches for the optimal minimum pressurerequired to adequately distend a patient's nasal pharyngeal airway. Byrendering the system portable, a large percentage of OSA victims can becost-effectively treated in their homes, thus reducing the overcrowdingin expensive hospital sleep centers. Optimal minimum pressure is usedbecause higher pressures increase the likelihood of side effects (e.g.,daytime rhinitis), and reduce the likelihood of patient compliance. Apatient's compliance in regularly using the system is a significantconcern inasmuch as the system is a portable device used at thepatient's home without the supervision of a hospital sleep centerspecialist.

To address the need for a practical device which will further enhancepatient compliance, transducers must not be placed on or in thepatient's body. Rather, all information used for automatic pressureadjustments is derived by continuously measuring airflow from thepressure generating source (blower) to the nasal mask. Airflow ismeasured quantitatively by a pneumotachograph interposed between thepressure generating source and the mask. This continuous measure ofairflow provides a feedback signal for the adaptive control system tomaintain a desired level of output pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will become moreapparent from the following detailed description of preferredembodiments when read in conjunction with the accompanying drawings,wherein like elements have been designated by like numerals and wherein:

FIG. 1a shows an exemplary embodiment of an auto-CPAP system with anadaptive control feature;

FIG. 1b shows a conceptual diagram illustrating an operator of theadaptive control system;

FIG. 2 shows a graph of characteristic features and upper airwayresistance versus mask pressure for one particular mechanical conditionof a simulated pharyngeal airway;

FIG. 3 shows a graph of characteristic features and upper airwayresistance versus mask pressure for a second mechanical condition of thesimulated pharyngeal airway;

FIG. 4 shows a graphical representation of decision criteria modifiersused in conjunction with an exemplary embodiment of an adaptivecontroller;

FIG. 5 shows an exemplary implementation of testing and non-testingmodes of operation;

FIG. 6 shows a display of pressure (PM), microphone signal (sound), O₂saturation (O₂ Sat) and body position (L: left side; R: right side; S:supine) in an OSA patient being treated with auto-CPAP;

FIG. 7 shows a general flow chart of overall operation of a preferredembodiment; and

FIGS. 8a-c show a portion of the FIG. 7 flow chart in greater detail.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 1. Overview

In accordance with a preferred embodiment, the present invention relatesto an auto-CPAP system for adaptively providing continuous positiveairway pressure in an upper airway system (e.g., pharyngeal airway of apatient). The auto-CPAP system performs detection, analysis, anddecision-making functions.

2. System Description

As illustrated in FIG. 1a, an exemplary system is generally labelled 2and includes means for detecting airflow from the CPAP device. The meansof detecting airflow is shown in FIG. 1a to include a pneumotach 8 formeasuring instantaneous airflow. The pneumotach includes a pressuretransducer and an amplifier for generating an electric signalproportional to airflow.

Airflow is used to assess the respiratory and dynamic mechanicalcharacteristics of a patient's pharyngeal airway (PA) during sleep andto adjust the therapeutic CPAP pressure as required. Total airflow isthe sum of non-respiratory and respiratory airflow. The non-respiratoryairflow corresponds to a bias flow plus system leaks.

Respiratory airflow typically corresponds to patient breathing and hastwo sequential, tidal components: one caused by inhalation and anothercaused by exhalation. This tidal airflow is phasic and therefore allowsthe onset of inspiration and the onset of expiration to be identified.The onset of inspiration corresponds to the time at which total airflowbegins to exceed non-respiratory airflow. The onset of expirationcorresponds to the instant when total airflow is less than thenon-respiratory value.

Using the non-respiratory airflow, airflow peaks and mean inspiratoryairflow can be determined. Because the onset and termination ofinspiration are identifiable, parameters related to the shape of a timeprofile of inspiratory flow can also be determined. In a preferredembodiment, a degree of roundness and flatness of the inspiratoryprofile are determined as will be described later.

The measurement of airflow and subsequent determination of aninspiratory airflow profile are used to control CPAP in accordance withthe present invention. When the level of nasal CPAP that produces themaximal distention with the minimum pressure is abruptly reduced insleeping patients suffering from OSA, the pharynx is observed tocollapse and the pharyngeal resistance increases accordingly. Thesechanges in upper airway resistance induce changes in peak inspiratoryairflow and profile shape with little change in airway pressure belowthe obstruction. Accordingly, changes in airflow resistance can beinferred from changes in the inspiratory airflow.

Further progressive reductions in nasal pressure lead to progressivecollapse of the pharyngeal airway which-severely reduces inspiratoryairflow and causes flow limitations (i.e., increased airflowresistance). As described herein, a flow limitation is a situation whereairflow rate is constant and independent of driving pressures.Similarly, progressive increases in nasal pressure lead to smallerdecrements in airflow resistance as the pharynx widens and reaches thelimits of its distensibility. The collapsible behavior of the pharyngealairway in response to progressive pressure changes provides a frameworkfor determining an optimal therapeutic CPAP in accordance with thepresent invention.

Accordingly, a preferred embodiment includes means for generatingpressure in the upper airway system in response to detected airflow.This pressure is adaptively adjusted to apply an optimal therapeuticnasal pressure. As illustrated in the exemplary FIG. 1a embodiment, apressure generating means is represented generally as a CPAP device. TheFIG. 1a pressure generating means includes a known, electronicallycontrollable pressure generating device 4, such as the commerciallyavailable “Tranquillity Plus” manufactured by Healthdyne Technologies.

The pressure generating device is a computer controlled pressuregenerator which supplies a commanded pressure to the pharyngeal airwayof a patient via a nasal mask 6 worn by the patient. Because the nosemask used has an exhaust port, a bias flow is present which increases atincreasing pressure. This bias flow constitutes a portion of thenon-respiratory airflow as mentioned previously.

Further, the FIG. 1a embodiment includes means for adaptivelycontrolling the pressure generating means in response to the airflowdetecting means to automatically provide optimal CPAP. Such a feature isgenerally illustrated in FIG. 1a as an A/D converter 10 and an adaptivecontrol system 12. The adaptive control system 10 is shown as a computer(e.g., an IBM 80386 compatible) and interface which communicates adesired pressure to the CPAP device.

During a testing mode of the FIG. 1b auto-CPAP system, the pressure inthe pharyngeal airway is changed frequently. Presently the pressure ischanged by sending the value of the actual pressure value via a serialport to an interface box of the adaptive control system which includes aZ-80 based microcomputer board (e.g., Micromint's BCC52computer/controller). The interface box reads the pressure and thenconverts it to a format (e.g., four line parallel input) for the normalremote control input into the pressure generating device.

Generally speaking, the adaptive control system generates an optimaldesired (i.e., command) pressure by averaging airflow data over apredetermined period of time, partitioning airflow data into respiratoryand non-respiratory components, identifying periods of inspiration andexpiration using the non-respiratory component, and extractinginformation or features from airflow data. Using this information, theadaptive control system identifies a critical pressure (P_(crit)) atwhich a significant obstruction occurs during inspiration. Moreparticularly, P_(crit) corresponds to a lower limit of mask pressureassociated with a significant decrease in peak inspiratory airflowand/or significant (i.e., critical) airflow limitation. Afterdetermining P_(crit), the adaptive control system identifies an optimum(i.e., minimum) effective CPAP (P_(opt)) for eliminating the obstructionduring inspiration.

The adaptive control system identifies P_(crit) and decides upon P_(opt)using a series of test perturbations in the mask pressure. Results ofthe tests are evaluated by examining inspiratory airflow. P_(opt) iscontinuously updated during testing periods which are initiatedthroughout the night to account for changes in the patient's sleepstages and sleeping position.

Because a testing period is used to update P_(opt), the adaptive controlsystem also decides when to test the pharyngeal airway, and when tocontinue or to stop testing. Further, the adaptive control system (1)manages overall operation to optimize its own performance, and (2)monitors potential airflow measurement errors to accurately measureupper airway performance as will be described below.

Airflow changes and airflow profile changes in the upper airway systemhave been determined to be directly related to intra-pharyngealpressure. By determining upper and lower limits of pharyngeal resistancefrom changes in airflow during a testing period, P_(opt) can bedetermined for any patient at any time. Accordingly, the adaptivecontrol system searches for P_(opt) between a lower airflow limitation(P_(crit)) and an upper limit (full distention of the airway).

Operating within these relative limits ensures reliable assessment ofthe pharyngeal airway and an accurate determination of P_(opt). Becauseairflow varies widely among patients and, for any particular patient,varies with sleep stage, P_(opt) can not be determined by comparingairflow measurements with ideal or predicted standards.

Generally speaking the computer 10 of FIG. 1a conceptually includes fourbasic components for performing the aforementioned testing andnon-testing control. As shown in FIG. 1b, these four basic componentsare an operator, a feature extractor, a testing protocol, and longtermmemory.

a. Operator

The adaptive control operator is an overseer that has access toinformation of the feature extractor at all times, decides when and whennot to enter the testing protocol, controls the flow of information toand from longterm memory, and maintains optimal performance andreliability. Decisions are made by the operator to ensure that theadaptive control system operates within predetermined operating limitsso that accuracy is maintained.

The normal operating limits for the adaptive control system are based onrules of operation. These rules of operation ensure that so calledperformance indices are within predetermined physiological ranges, andthat a respiratory phase threshold detection mechanism system isfunctioning efficiently. Further, these rules are used by the adaptivecontrol system to make decisions, such as when to exit a testing periodor when to return to a testing period.

To ensure operation within predetermined physiological limits, the rulesare designed to have the adaptive control system operate whenever thereis (1) a low to moderate level of variation in respiratory features, (2)no hypoventilation and (3) no apnea. Further, the rules are designed tohave the adaptive control system operate with a threshold detectionmechanism that adjusts for leaks in the CPAP-to-patient system. Thethreshold detection mechanism is used by the feature extractor todetermine changes in phase of respiration.

For purposes of the present discussion of preferred embodiments, a highvariation in the respiratory features is defined as a variationcoefficient value of 0.3 or more for four or more specified features(e.g., time of inspiration (T_(i)), total time of breath (T_(tot)), meaninspiratory airflow (V_(m)), peak inspiratory airflow (V_(p)), andRoundness) for a set of 5 or 10 breaths depending on whether it is in atesting or a non-testing mode, respectively; hypoventilation is definedas five (5) consecutive breaths with V_(m) less than 40 percent of thepredicted awake supine V_(m); and apnea is defined as a 10 secondsduration of no change in respiratory phase as determined by the leakadjusted threshold detection mechanism. The threshold detectionmechanism is judged to not be completely adjusted to the actualnon-respiratory flow when a significant increase or decrease in thecalculated non-respiratory flow (0.03 L/sec) has occurred over a periodof five (5) breaths.

Satisfaction of these rules and proper adjustment of the thresholddetection mechanism are criteria used by the adaptive controller indeciding whether or not to enter a testing mode. If these rules are notsatisfied during a non-testing period, either a subsequent testingperiod is delayed or the CPAP is adjusted or both. If these rules arenot satisfied during a testing period, the testing ceases and there is areturn to the previous P_(opt), or to a pressure previously set by anoutside source, whatever is higher.

As mentioned above, the operator is an overseer which decides when toenter a testing mode. Decisions made by the adaptive control system(e.g., when to test and when to discontinue testing) are based ondynamic characteristics, or performance indices, of the pharyngealairway during the non-testing and testing periods. During non-testingand testing periods, the adaptive control system continuously monitorsbreathing variations, hypoventilation, apnea, and signs that thethreshold detection mechanism has not been properly adjusted for leaks.

(1) Non-Testing Mode Periods

The adaptive control system operates in one of two basic modes: anon-testing mode (n-TM) and a testing mode (TM). Throughout the testingand non-testing modes, characteristics of the upper airway arecontinuously detected and evaluated by the feature extractor. In thenon-testing mode (i.e., non-testing period), results generated by thefeature extractor are used to determine if and when to delay testing, tooptimize rules of operation, and to identify deteriorating changes inairflow.

While in the non-testing mode, the FIG. 1 auto-CPAP system monitors theinformation from the feature extractor. This information is used todetermine the presence of large variations in breathing frequency,hypoventilation, apnea, and signs of unadjusted leaks which would affectthe nRV. Testing under these conditions could lead to erroneous results.Therefore entering into the testing mode may be delayed. Hypoventilationmay also occur during this period if mask pressure is too low.

(2) Testing Mode Periods

When the adaptive control operator decides to redetermine P_(crit) andP_(opt), then the testing mode is executed in accordance with thetesting protocol. As in a non-testing period, the operator hascontinuous access to the information from the feature extractor during atesting period to determine if it should continue to test for P_(crit)and P_(opt).

When the FIG. 1a auto-CPAP system enters the testing mode, a specifictesting protocol of pressure perturbations is followed. Prior toidentifying P_(opt), the testing protocol is only interrupted if a largebreathing variation, an apnea or hypoventilation is detected. Theresults from the non-testing mode and the testing mode are retained inthe longterm memory.

b. Feature Extractor

The feature extractor (FE) is the center for continuous acquisition andanalysis of data. For example, the feature extractor generatesperformance indices in response to respiratory airflow data. Theseperformance indices are a measure of the pharyngeal airway's dynamicstate and are used by the operator for decision making in both thetesting and non-testing modes. In alternate embodiments, additionalsignals (e.g., monitoring signals related to oxygen saturation andsound) can be input to the feature extractor to assist in the continuoussensing of dynamic characteristics of the pharyngeal airway.

The feature extractor has two basic functional modules: a dataacquisition module and a respiratory cycle analysis (RCA) module. In theexemplary FIG. 1a embodiment, data acquisition of the input signals(e.g., airflow) occurs via the 12 bit analog-to-digital converter 10(e.g., Data Translation DT2821) every 8 msec.

The digital values are then passed into an RCA module where eightconsecutive values are averaged to produce a single low pass filteredaverage value every 64 msec. Each 64 msec average value is thencontinuously analyzed in the RCA module for phase of respiration, apnea,and breath features. An important characteristic of the RCA module isthat it continuously analyzes the airflow signal to identify therespiratory component (i.e., respiratory volume, RV) and thenon-respiratory component (i.e., non-respiratory volume, n-RV).

Performance indices generated by the RCA module are updated continuouslyas follows, where the asterisks indicate a real time occurrence of anupdate for the feature listed:

During During Inspiration Expiration Respiratory * * (continually) phaseEnd of Breath * (end of expiration) RCA Abnormalities * * (Excessiveleak detection  error and future error  reporting) Apnea * * BreathFeatures: T_(i) * (time of inspiration) T_(e) *  (time of expiration)T_(tot) *  (total time of breath) Vol_(i) * (inspiratory volume)Vol_(e) *  (expiratory volume) V_(m) * (mean inspiratory  airflow)V_(p) * (peak inspiratory  airflow) Flatness * (measure of inspiratory flatness) Roundness * (measure of inspiratory  roundness)

As mentioned previously, an optimum pressure is determined by evaluatingthe effects of incremental pressure perturbations on inspiratoryairflow. Accordingly, the RCA module is designed to continuously reportbreath changes in upper airway state (i.e., to identify respiratoryphase and end of breath conditions based on extracted features). Abreath is defined as an inspiratory period followed by an expirationperiod. Therefore, an end of breath condition is updated at the end ofexpiration.

When the RCA module detects a problem, then an RCA abnormalitiescondition is set. For example, an abnormal condition is set eitherduring inspiration or expiration when excessive flow leakage isdetected. Further, the RCA module is designed to continuously reportdetection of apneas based on extracted features.

The breath features listed above are the dynamic physiologicalcharacteristics of the pharyngeal airway. Their variation, especially incombination, are excellent measures of the pharyngeal airway behavior.Values of T_(i), T_(e), T_(tot), Vol_(i), Vol_(e), V_(m) and V_(p)(defined in the above table) are physiologically self explanatory breathfeatures. Flatness and roundness values are breath features which aredeveloped as measures of inspiratory airflow. The flatness and roundnessvalues are used in accordance with preferred embodiments to identifypharyngeal airway behavior.

For purposes of the present discussion, flatness is defined as therelative deviation of the observed airflow from the mean airflow. In apreferred embodiment, individual values of airflow are obtained between40% and 80% of the inspiratory period. The mean value is calculated andsubtracted from the individual values of inspiratory flow. Theseindividuals differences are squared and divided by the total number ofobservations minus one. The square root of this product is used todetermine a relative variation.

The relative variation is divided by the V_(m) to give a relativedeviation or a coefficient of variation for that breath. This measure ofairflow therefore represents a measure of flatness over the mid-range ofinspiration. A relatively low value is used to indicate that inspiratoryairflow during mid-inspiration is relatively constant. The common causeof this is flow-limitation secondary to pharyngeal collapse. Thus, a lowvalue indicates the need for higher nasal CPAP pressures.

For purposes of the present discussion, the roundness feature suppliesinformation regarding the similarity between the normalized inspiratoryflow profile and a sine wave normalized for observed inspiratory timeand for observed peak flow. The airflow predicted from the sine wave,Vsine, is calculated from the following normalized sine wave equation:

Vsine=Vpeak*sine(F*π)

where Vpeak is observed peak flow and F equals the fraction ofinspiratory time elapsed. This equation for predicting sequentialairflow measurements is used when the ratio of peak flow to T_(i) isless than 1.1 and greater than 0.45. For values of the ratio greaterthan 1.1 the peak is estimated by multiplying T_(i) by 1.1, and forvalues below 0.45 the peak is estimated by multiplying T_(i) by 0.45.

The differences between consecutive values of observed inspiratoryairflow and that calculated from the sine wave equation value aresquared and summed, and then divided by the total number of points. Thesquare root of this product is then divided by the mean value of airflowfor that inspiration to give a normalized value for that breath.

Accordingly, the roundness index provides an estimate of the degree towhich the inspiratory airflow profile resembles a sine wave. As flowlimitation occurs or as the airflow signal becomes less sinusoidal, theroundness feature becomes larger. This indicates an increase in upperairway resistance and suggests that mask pressure may not be adequate.

FIGS. 2 and 3 illustrate a relationship between the characteristicfeatures V_(p), flatness, roundness, and upper airway resistance(R_(uaw)) versus mask pressure (P_(m)) The values presented are from astarling resistor model of the pharyngeal airway.

In FIG. 2, the pressure surrounding the collapsible tube (P_(s)) is 6cmH₂O, whereas in FIG. 3, the P_(s) is 10 cmH₂O. For values of P_(m)greater than P_(s), the features are represented by values which are attheir minimum (e.g., roundness) or maximum (e.g., flatness and V_(p)),with R_(uaw) being zero. For values of P_(m) below P_(s), as R_(uaw)rises the features dramatically rise or fall. These results indicatethat V_(p) and flatness are measures of flow limitation and roundness isa measure of increasing upper airway resistance.

To update the performance indices and other information presented in theabove chart, the RCA module includes the aforementioned respiratoryphase threshold detection mechanism (TDM). The threshold detectionmechanism detects the inspiratory and expiratory phase changes inairflow. The accuracy of the feature extraction is very dependent uponaccurate detection of the start of inspiration. In accordance withpreferred embodiments, the start of inspiration is ascertained solelyfrom airflow.

Basic assumptions in the threshold detection mechanism are thatinspiratory and expiratory volumes are approximately equal. Two factorsaffect the volumes causing them to be unequal. The volume of oxygenconsumed per unit time is normally greater than the volume of carbondioxide that is produced by the body. Further, breath-to-breathvariation in tidal volume and timing during sleep, as well as arousalwhich alters alveolar ventilation and exact expiration volume, canresult in a variation between inspiratory and expiratory volumes.

Normally the inspiratory tidal volume is 4% greater than the expiratorytidal volume. Over a 30 second period of quiet breathing, all variationscan be approximately averaged out of this ratio. Therefore, a resultantaverage respiratory flow can be used as a basis to estimate thebeginning of inspiration and to approximate non-respiratory flow.

When breathing without a bias flow, the actual start of inspiratory flowcan be detected when the airflow signal crosses a no-flow value. This isbecause the actual zero respiratory flow corresponds to the zero flowvalue.

The characteristic increase in slope of flow which marks the onset ofinspiration occurs even in the presence of non-respiratory flow.However, when a non-respiratory flow exists, pneumotach zero flow andzero respiratory flow are not the same as the non-respiratory flowconstitutes a continuous flow through the pneumotach.

Accordingly, when a bias flow is present, the non-respiratory componentof airflow must be determined to accurately identify the start ofinspiration and the derived features. As mentioned above, thenon-respiratory component is defined as that component of airflow notdue to normal respiratory airflow. This non-respiratory airflow includestwo components: the first is a known component due to the bias flow outthe exhaust port of the nose mask used for washing out end tidal CO2from the mask.

The second is a variable, unknown flow due to leaks around the mask orout of the patient mouth. The magnitude of this unknown flow iscalculated and is used to establish times when the feature extractor maybe in an unstable period such that information from the featureextractor may be inaccurate. More particularly, changes in the exhaustflow out of the mask due to pressure changes can be measured. A percentchange in exhaust flow due to a pressure change can then be used toestimate a similar percentage change in total non-respiratory airflowduring system operation.

The threshold detection mechanism for determining the onset ofinspiration and expiration is highly dependent on the determination ofthe non-respiratory component. If the computed non-respiratory componentis not within the specific range of the actual zero respiratory flow,then the resultant breath features are inaccurate. Corrections can bemade during a non-testing mode or a testing mode up to a maximum of 1.6L/sec and down to a minimum established by the known exhaust flow forthe present mask pressure.

When the value for the average respiratory flow approximates the actualzero respiratory flow in the presence of mask exhaust flow and leakflow, the start of inspiration can thus be estimated using the change inslope as a characteristic marker. A lower limit can be attached to thedetermination of the start of inspiration by using a rule thatinspiratory flow cannot occur below the known flow through the exhaustport. The following are exemplary preferred rules for estimating thestart of inspiration and expiration.

Onset of Inspiration:

1) Average respiratory flow is used to approximate a range of flow whereinspiration will mostly occur.

a. Average respiratory flow is primarily derived by an ongoing digitalaveraging of respiratory flow for a 64 sec window of time.

b. The mean value of four (4) successive estimations of the start ofinspiration are determined. These four mean values are averaged into theaverage respiratory flow with a weight of 32 secs.

2) A range around the average respiratory flow of −0.05 L/sec and +0.1L/sec is used to approximate the most likely occurrence of the start ofinspiration.

3) While in expiration and within the above range, the start ofinspiration is, in a preferred embodiment, Tentatively True when:

a. The slope of the present 64 msec flow sample is greater than 0.39L/sec and the slope of the previous 64 msec flow was below 0.30 L/sec,

b. the slope of the present 64 msec flow sample is greater than 0.39L/sec and the slope of the previous 64 msec flow was also greater than0.30 L/sec, or

c. the flow has exceeded the upper limit of the range, 0.1 L/sec abovethe average respiratory flow, and

d. the estimated start of inspiration is above

(1) the intentional leak for the present mask pressure during a P_(crit)search, and

(2) during a P_(opt) search, the average respiratory flow.

4) The start of inspiration is True when:

a. The slope remains above the upper limit of average respiratory flow(+0.1 L/sec), or

b. A minimum slope of 0.27 L/sec is maintained for 0.42 secs.

5) The actual start of inspiration is estimated to occur at the previousflow rate when the start of inspiration was Tentatively True.

Onset of Expiration:

1) The same flow rate that was estimated for the start of inspiration isused as the start of expiration.

The continuous adaptation of averaged respiratory flow to changes in thenon-respiratory flow (leaks) is made by several mechanisms. First, theaveraged respiratory flow value is continuously calculated as a movingaverage using the 64 msec value of flow over a 64 sec window of time.This method produces a very constant value of average respiratory flow.Second, the use of an averaged start of inspiration as a weighted factorin the calculation of the averaged respiratory flow results in quickeradaptation of the threshold detection mechanism to moderate leaks thatoccur over an approximate 32 sec period. In addition, the averagedrespiratory flow is re-initialized to the new mask pressure by: 1) priorto changing mask pressure, a ratio is found from the existing averagerespiratory flow divided by the present mask pressure, 2) the maskpressure is changed, then 3) the averaged respiratory flow isre-initialized to the new mask pressure times the ratio.

In the case of a sudden leak which could send the total airflow abovethe threshold detection mechanism, the result can erroneously bedetected as an apneic event even though respiratory changes areoccurring in the flow signal. In this case, when no change inrespiratory phase has occurred for five (5) seconds then the highest andlowest airflow values are searched for the remaining 5 seconds of the 10seconds limit for an apnea determination. Before an apnea condition isflagged true, if the difference between the highest and lowest airflowis greater than 0.3 L/sec, then an apneic condition is not flagged andthe averaged respiratory flow is re-initialized to the midpoint betweenthe highest and lowest airflow found in the last five (5) seconds of theapnea test. This last mechanism is designed to adapt to rapid changes innon-respiratory airflow due to leaks that may appear as an apnea to thefeature extractor.

Around the average respiratory flow, the method of detecting inspiratoryand expiratory flow minimizes the computational load in deciding phasechanges and maximizes the accuracy of the pattern recognition.

C. Testing Protocol

During testing periods, the adaptive control system first reducespressure and determines P_(crit). This constitutes a characteristiclower limit of mask pressure for a given state of the patient'spharyngeal airway (e.g., sleep stage, position, and so forth). Havingestablished this lower limit of pressure, the optimum higher pressurevalue P_(opt) is determined by progressively increasing intra-pharyngealpressure. The increases in peak inspiratory pressure and changes inshape of inspiratory airflow profile are recorded and used to identifyP_(opt).

The determination of P_(crit) during a testing period is termed theP_(crit) search. The subsequent determination of P_(opt) during atesting period is termed the P_(opt) search. Each search consists of aprogressive series of incremental changes in mask pressure (i.e., stepdecreases for P_(crit) and step increases for P_(opt)).

During a preferred search to identify P_(crit) there are two types ofpressure decreases that are performed when normal rules of operationhave been satisfied. The first is a 4 cmH₂O decrease in mask pressure(P_(crit) scan) which is used to test the pharyngeal airway forsignificant collapsibility before a P_(crit) search actually begins. Thescan begins at a holding pressure (Ph) used during the precedingnon-testing period. If the information from the feature extractor duringthe P_(crit) scan indicates that a significant airflow limitation hasoccurred, then a limit to subsequent P_(crit) searching is set (4 cmH₂Obelow the holding pressure). This limit prevents excessive searching forthe pressure which produces insignificant flow limitation.

The second type of pressure decrease that is performed when the rules ofoperation have been satisfied is referred to herein as the P_(crit)search. A P_(crit) search is performed after a P_(crit) scan. During theP_(crit) search, the pressure perturbations are a series of 2 cmH₂Odecreases in mask pressure.

A test for P_(crit) during a P_(crit) search is repeated untilpredetermined decision criteria have been met (i.e., changes in peakinspiratory airflow and/or profile shape features detected by thefeature extractor exceed predetermined decision criteria) or until alimit to the P_(crit) search set by the P_(crit) scan is encountered.Each P_(crit) test is initiated with a pre-test period which is followedby a single breath test period and a five breath post-test period.However, when the decision criteria for the P_(crit) search have beensatisfied during the single breath test, there is no post-test period.

The P_(opt) search is a series of tests or increases in mask pressure(e.g., 2 cmH₂O) which is initiated after P_(crit) has been determined.The search for P_(opt) involves finding the mask pressure at which thepeak flow and the flow profile do not improve after a 2 cmH₂O increasein mask pressure. Thus, the minimum effective CPAP pressure representsthat mask pressure at which there is no improvement in the flow profileafter a worsening in the flow profile.

Each P_(opt) test is initiated with pre-test similar to that of aP_(opt) pre-test. A single breath test period and a five breathpost-test period follow the pre-test. In a P_(opt) search, the post-testis used to detect an unadjusted non-respiratory flow error.

An unadjusted non-respiratory flow error is detected in the P_(opt)search when the non-respiratory flow of a 5th breath detected during thepost-test is greater than that of a 1st breath detected during thepost-test (e.g., by 0.03 L/sec as described above with respect to thefeature extractor) following an increase in mask pressure. If anunadjusted non-respiratory flow error was not detected, then the fivebreaths of the post-test period are use as pre-test values and a singlebreath test is performed immediately after what was the post-testperiod. If an unadjusted non-respiratory flow is detected then apre-test period is performed unless at least one P_(opt) test has beenperformed. If one P_(opt) test has been performed and there is anunadjusted non-respiratory leak then P_(opt) is deemed to have beenfound and testing is stopped. The pre-test period after a post-test inthe P_(opt) search allows for non-respiratory adjustments.

A P_(opt) search continues provided normal rules of operation are metuntil predetermined decision criteria for a minimum effective CPAP havebeen met. If an unadjusted non-respiratory error has occurred and atleast one P_(opt) test has been performed, then P_(opt) is determined tohave been found at the present pressure and the current testing mode isexited.

In any test, if the decision criteria for a flow alone condition wasexceeded (P_(crit)) or not exceeded (P_(opt)), then the test isrepeated. A flow alone condition corresponds to a relatively largechange in peak airflow with little or no relative change in roundnessand/or flatness. If an apnea, hypoventilation or respiratory variationerror is detected during the testing, the testing mode is exited and thesystem goes directly to the holding pressure of the previous non-testingperiod.

The decision criteria for P_(crit) are considered to have been satisfiedif a relative change in extracted features exceeds the predetermineddecision criteria (DC) in any one of four ways: (1) difference betweenfeature values extracted during a first breath test and currentlyestablished pre-test feature values exceed the DC; (2) differencebetween feature values extracted using an average of 4th and 5th breathsdetected during the post-test (post-test average) and currentlyestablished pre-test feature values exceed the DC; (3) differencebetween feature values extracted during subsequent single test breathsand the initial pre-test feature values previously established duringthe initial pre-test exceed the DC; or (4) difference between featurevalues extracted during subsequent post-tests and feature values of theinitial pre-test exceed the DC. The detection of P_(crit) using thecomparisons of (3) and (4) above is referred to herein as a trend test.While comparisons similar to (1) and (2) above are used to identifyP_(opt), the trend test comparisons are used only to determine P_(crit).

More particularly, the trend test is used exclusively in the P_(crit)search to detect a progressive decrease in the flow profile over theP_(crit) search that may not show up during any one single breath testor post-test. As described above, the trend test uses the initialpre-test features (e.g., five breath average) as the template forsubsequent comparisons during tests (3) and (4).

In an exemplary embodiment, a test is true during a P_(crit) search ifrelative changes in the V_(p) feature and the flatness feature orrelative changes in the V_(p) feature and the roundness feature haveexceeded the DC. Similarly, during a P_(opt) search, if relative changesin the V_(p) feature and the flatness feature or relative changes in theV_(p) feature and the roundness feature changes have not exceeded theDC, the test is true.

As described above, the process of decision making during the search forP_(crit) and P_(opt) is based on a comparison of relative changes inextracted features with the actual DC for each feature. The DC aredetermined by modifying significant decision criteria (SDC) withsignificant decision modifiers (SCDM) that depend upon the maskpressure.

Prior to starting the auto-CPAP system, SDC are established for eachfeature for both the P_(crit) and P_(opt) search. Exemplary SDC valuesare:

P_(crit) P_(opt) V_(p): (flow only) 0.24 0.20 V_(p): (combine 0.21 0.20with below) Flatness: 0.24 0.20 Roundness: 0.40 0.20

The SDC are preselected values for the relative change in peakinspiratory flow and/or profile shape indices. The SDC are the basicstandards used for comparison with observed changes in peak inspiratoryflow and/or profile shape indices during the P_(crit) and P_(opt)searches. The SDCM is a factor which varies as a function of maskpressure (FIG. 4) and modifies the DC values so that the actual oroperational decision criteria varies in accordance with statisticalprobability.

The above SDC's for each feature and search are modified depending uponthe mask pressure at the time of the test by multiplying the SDC withthe decision criteria modifiers (SDCM). FIG. 4 illustrates exemplarymodifiers used on the SDC. In FIG. 4, the abscissa corresponds to maskpressure values while the ordinate corresponds to SDCM values.

Two SDCM curves are presented in FIG. 4. One is for the P_(crit) searchand the other is for the P_(opt) search. During the P_(crit) search, asthe mask pressure is reduced, the modifier becomes less and the criteriato be exceeded becomes less. Therefore, it becomes easier to findP_(crit) with less test pressure. Conversely, during the P_(opt) search,the SDCM becomes higher and it becomes easier to find a change less thanthe modified SDC.

The DC for P_(crit) and P_(opt) are derived by a formula thatincorporates the SDC and the SDCM. The SDC for each performance indexand desired combinations of SDC's are set for P_(crit) and P_(opt). TheDC formula for each performance index is calculated as follows:

DC=SDC×SDCM

As an adaptive control system, the auto-CPAP has several mechanisms tomaintain and improve optimal performance and reliability. One suchexemplary mechanism is the modifying the SDCM to establish an optimumrange where the likelihood or probability of finding P_(crit) andP_(opt) for each patient is highest. Another mechanism is the continuousadjusting of the estimation of the non-respiratory flow as describedpreviously.

This latter mechanism has several features that act to optimize thedetection of inspiration. The first is a weighting of estimations of thestart of inspiration into a moving average of airflow. The second is theimmediate re-initializing of the moving average of airflow when the maskpressure is changed and a leak condition has occurred but a change inrespiration is not detected. Another exemplary optimizing feature isweighting the performance indices and their combinations to increaseaccuracy of establishing P_(crit) and P_(opt).

The single best manner of maintaining reliability is to operate withinthe normal rules of operation. The rules can be modified if a particularproblem is repeatedly encountered (e.g., leaks) to ensure performanceindices are correct. The SDC, SDCM, and the normal operating rules canalso be modified by additional external inputs. For example, the FIG. 1asystem can operate with known monitors that are used in the diagnosis ofOSA. One such commercially available oxygen saturation and snoringmonitoring device is a portable sleep apnea monitor known as MESAM,available from Healthdyne Technologies of Marietta, Ga., USA. Inputsfrom this, or other monitors, can be input to the FIG. 1a adaptivecontrol system along with the airflow signal.

As will be described in greater detail during a discussion of systemoperation below, a search for P_(crit) begins with the scan protocol. Asmentioned above, an exemplary scan is a single 4 cmH₂O decrease in maskpressure. This single breath decrease is preceded by 5 breaths. The maskpressure during the 5 breaths which precede the pressure drop of a scanis either the holding pressure during the non-testing period, or thelast test pressure during a P_(crit) search if the scan protocol isrepeated during a P_(crit) search.

The average values from the features during the pre-pressure drop of ascan are used as control values during the scan. If the comparisonbetween the 5 breath average and the post pressure drop during a scan issignificant (as determined by the DC), the system records that the scanwas significant and the post scan pressure becomes the limiting pressureduring the P_(crit) search (4 cmH₂O below present holding pressure).

The search protocol begins with the search for P_(crit) at the sameholding pressure as the preceding scan (i.e., prior to the 4 cmH₂Odrop). The search protocol begins with a pre-test during which 5 breathsprior a pressure decrease are averaged and used as controls forcomparisons during subsequent single breath tests and post-tests.Following the pre-test breaths, the pressure is dropped 2 cmH₂O and thesubsequent inspiratory breath features are collected.

If the breath features during the decrease in pressure did not exceedthe DC set for this level of mask pressure, then the mask pressure isleft unchanged and a post-test period begins consisting of 5 breaths.The fourth and fifth breaths of this post-test period are averaged(i.e., post-test average) and the average is tested to see if itexceeded the same DC of the single breath test. If the DC is exceeded ineither the single breath test or the post-test average, then the maskpressure is returned to the mask pressure set during the pre-test periodand a P_(opt) search is initiated.

If neither the single breath test nor the post-test average exceeded theDC, then another test is performed, in this case a P_(crit) test.Accordingly, during a subsequent single breath test and post-test, atrend test will be used to compare extracted features with features ofthe initial pre-test average. These comparisons are performed inaddition to comparisons of extracted features with the current pre-testaverage as discussed above.

In an exemplary embodiment, if a second cycle of a P_(crit) searchpre-test, single breath test, and post-test does not exceed the DC, orif the previous P_(crit) scan was significant but the limiting pressurewas not reached, then the scan protocol is repeated at the previoussearch mask pressure. This basic scan-search combined protocol isrepeated until the lowest mask pressure is reached or until thecomparisons exceed the test criteria. For example, if the initial scanwas not significant and P_(crit) has not been detected after twoincremental pressure decreases, another scan will be performed. In thisscan, an additional 4 cmH₂O pressure drop is introduced (i.e., total 8cmH₂O drop). The aforementioned P_(crit) search is then repeated.

An exemplary search protocol for P_(opt) is slightly different than thesearch used to identify P_(crit). A scan is not used in the testingprotocol to identify P_(opt). Further, during a preferred P_(opt)search, a pre-test series of 5 breaths precedes an incremental increasein mask pressure. Further, the trend tests used to identify P_(crit) arenot used to identify P_(opt). The P_(opt) search protocol consists of 5pre-test breaths, a 2 cmH₂O step increases in pressure, and an optional5 post-test breaths if an unadjusted non-respiratory error was detectedand if it was the first P_(opt) test. This P_(opt) protocol is repeateduntil no significant differences exist between V_(p) and/or profileshape indices of the pre-test relative to the single breath test and thepost-test, or until there is a unadjusted non-respiratory error and atleast one P_(opt) test.

FIG. 5 illustrates exemplary scan and search pressure perturbationprotocols in the search for P_(opt). FIG. 6 shows an exemplary displayof pressure, sound, oxygen saturation and body position in a patientbeing treated with auto-CPAP in accordance with a preferred embodiment.In FIG. 5, the x-axis is a compressed time scale while the y-axis ismask Pressure (P_(m)) from 0 to 20 cmH₂O. CPAP 1 period is a non-testingperiod where the Ph was 12 cmH₂O.

In the first scan protocol, scan 1 is shown with a 5 (5 breaths) abovethe line indicating the pressure of the pre-single breath test (1 belowthe pressure line). The pressure drop for the scan 1 was from 12 to 8cmH₂O and held for one complete inspiratory period and then returned tothe previous holding pressure. Scan 1 was not found to be significant.Therefore, the P_(crit) scan limitation is not set, and P_(crit) search1 is initiated.

The first P_(crit) search protocol begins with 5 breaths at the sameholding pressure previous to the scan. As shown in FIG. 5, the P_(crit)search was not significant for the next two successive decreases in maskpressure, and so a second scan protocol, scan 2, was initiated at themask pressure of 8 cmH₂O. The second scan of the second scan protocolwas significant (i.e., going from 8 to 4 cmH₂O). Thus, the scan wasjudged significant and the limiting pressure was set as 4 cmH₂O. Duringa subsequent P_(crit) search, the first decrease in pressure, search 2A,did exceed the DC for the mask pressure of 6 cmH₂O. The search forP_(crit) was therefore ceased and P_(crit) was set to 6 cmH₂O.

The mask pressure was then returned to 8 cmH₂O and the search forP_(opt) was started, search 2B. The first test for P_(opt) resulted inno significant changes in the inspiratory features (i.e., no change inincrementing 2 cmH₂O, from 8 to 10 cmH₂O). Thus, the P_(opt) search wasstopped and the new holding pressure was set at 1 cmH₂O less than thelast single breath test and post-test pressure (i.e., 9 cmH₂O).

d. Long Term Memory

The long term memory stores specific information for use by thephysician or the Sleep Laboratory for diagnostic or for follow-uptherapeutic applications. In addition to recording upper airway systemcharacteristic features during system operation, stored information canbe assembled to identify the patient's use of the auto-CPAP system (homeuse) or in diagnostic or therapeutic studies. This information can beused by the physician to assess the integrity of results obtained duringhome or lab use of the system.

3. System Operation

A more detailed discussion of overall system operation and inparticular, implementation of a preferred testing protocol, will now beprovided. FIG. 7 shows a general flow diagram of an exemplary systemoperation. In block 50, the system is powered for use. In block 52, thesystem is initialized. This step includes setting all ports as desiredon the adaptive control system 12.

Following start-up and initialization, the adaptive control systemoperator enters a non-testing mode as indicated by block 54. While inthe non-testing mode, the operator continuously evaluates whether atesting mode can be entered to update P_(crit) and P_(opt). In apreferred embodiment, a test mode can be entered only after apredetermined number of breaths have occurred without the detection ofbreathing instabilities (e.g., apnea, hypoventilation or variablebreathing).

An interval count is used to keep track of the number of breaths. Eachinterval count represents ten breaths. The interval count is compared tothe maximum interval limit (block 56) which can range, in a preferredembodiment, from a minimum of two counts to a maximum of five counts.After five counts (i.e., 50 breaths), a decision is automatically madeto enter a testing mode even though breathing instabilities may exist(block 56).

After two intervals (i.e., 20 breaths), if all rules are satisfied(i.e., no breathing instabilities) the testing protocol of block 58 isinitiated. However, if all rules are not satisfied (i.e., apnea,hypoventilation and/or variable breathing is detected by decision blocks60, 62 and 64), testing is delayed by increasing the maximal intervallimit up to three times (block 66) and the non-test interval count isincremented (block 68). The system then remains in the non-testing modefor 10 more breaths.

Where breathing instabilities have been determined even after 50consecutive breaths, the consistency of the instability is deemedsufficient to warrant initiation of a test mode. Thus, at any non-testinterval count equal to or greater than maximum interval limit, atesting mode can be initiated (block 58).

During a testing mode, a search is first made for P_(crit). OnceP_(crit) has been determined, a search is made for P_(opt). In thetesting mode, the system continuously checks for apnea, hypoventilationand/or variable breathing (blocks 70, 72 and 74). If any of theseinstabilities are detected (or any changes in these values are detectedin the case where the test mode was entered after 50 breaths), testingis stopped (block 76). The non-test interval count is reset to zero(block 78) and the system returns to a non-test mode.

At this point, because neither P_(crit) and P_(opt) have beendetermined, the system uses the last, accurately determined P_(opt)during the non-test mode (block 76). Alternately, the system can be setup such that block 76 represents use of a P_(opt) determined as anaverage of prior (e.g., two) accurately determined P_(opt). In yetanother embodiment, an experimentally determined P_(opt) previouslyspecified by a sleep lab specialist can be used.

Assuming the test mode is not interrupted, a search for P_(crit) is thenperformed (block 80). During, this search, the system continuouslymonitors breathing (loop 82) to detect instabilities which may influenceaccurate determination of P_(crit). Once P_(crit) has been determined, asearch is made for P_(opt) (block 84). Again, during the search forP_(opt), continuous monitoring of breathing is performed (loop 86). Whenboth P_(crit) and P_(opt) have been found, the system resets thenon-test interval count to zero (block 78) and returns to a non-testmode (block 54).

FIGS. 8a-c show a more specific flow chart of an exemplary testingprotocol represented by block 58 in FIG. 7. The steps of the FIG. 8 flowchart are first executed to determine P_(crit). Afterwards, the FIG. 8steps are repeated to identify P_(opt).

In FIG. 8a, once a test mode has been initiated (i.e., start block 90),a decision is made whether a scan test flag has been set TRUE to triggera P_(crit) scan (block 92). Assuming that the testing mode has just beeninitiated such that the scan test flag has not yet been set FALSE, ascan test is performed to initiate a P_(crit) scan (block 94).

The scan test is used to determine whether a pressure drop of, forexample 4 cmH₂O, from the pressure generating means results in asignificant flow limitation. Throughout the scan test (and throughoutall stages of the testing mode), a continuous monitoring of breathing isperformed to detect instabilities (i.e., blocks 70, 72 and 74 of FIG.7).

In block 96, assuming no breathing instabilities were detected, adecision of whether the scan was significant is performed. For thispurpose, five breaths are collected and their features averaged. Theaverages are compared to features detected after the 4 cmH₂O drop).

If the scan was significant, a scan significant flag is set TRUE(indicating that a lower limit for P_(crit) has been established) andthe pressure used prior to the 4 cmH₂O drop is used to initiate aP_(crit) search. To determine whether a scan was significant, a value of1.1×DC is used (rather than the aforementioned FIG. 4 SDC and SDCM)since a relatively large pressure drop is used for the P_(crit) scan.

In block 98, a pressure drop limit of, for example, 2 cmH₂O isestablished to search for P_(crit). In block 100, a pre-test TRUE flagis set to initiate a pre-test (block 104) and a scan test FALSE flag isset to indicate completion of a scan test.

Where the scan was not determined to be significant at block 96, thepre-test flag is nevertheless set TRUE and the scan test is set FALSE.Because the scan significant flag is not set TRUE, another scan can besubsequently initiated if P_(crit) is not identified in response to twoincremental pressure decreases.

Upon completion of the scan, a return (block 102) is made to start block90. Since the scan test flag has been set FALSE, the system sequences topre-test (block 104).

During a pre-test, information associated with five breaths is collected(block 106) for comparison with single breath information collectedduring a subsequent single breath test. Upon collecting the five breathinformation, the averages of the breaths are calculated and if this wasthe first or initial pre-test period the averages are saved for latercomparisons in the trend tests. Following block 106, a variablebreathing test is performed (block 108) during which collectedinformation regarding five breaths is examined for variations. If thevariations are significant (decision block 110), a variable breathingflag is set TRUE (block 112). Further, a pre-test completion flag is setFALSE and a change pressure flag is set TRUE (block 114).

The setting of the flags in block 114 causes the system to perform achange pressure sequence (block 118) following a return from thepre-test (block 116) to the start block 90. Where no variable breathingwas detected at block 110, the variable breathing flag is not set TRUEprior to initiation of a change pressure sequence.

Once the scan test and pre-test flags have been set FALSE, the systemsequences from block 90 to a change pressure sequence (block 118). In achange pressure sequence, the system awaits a start of expirationsubsequent to the five breaths of the pre-test (block 120). The pre-testis similarly performed during a subsequent P_(opt) search which isinitiated after P_(crit) has been determined.

At the onset of expiration during a P_(crit) search (as determined atblock 122), the remote holding pressure (P_(h)) from the pressuregenerating means is reduced, for example, 2 cmH₂O (block 124). During aP_(opt) search, the remote holding pressure is increased by, forexample, 2 cmH₂O (block 126). After a pressure change has been effected,a change pressure flag is set FALSE, and a single breath test flag isset TRUE (block 128) to initiate a single breath test (block 132)following a return block 130.

During a single breath test, the onset of a next sequential expirationis detected (block 134). Once this subsequent expiration has beendetected, the feature extractor computes roundness and flatness (block136) for the pressure set during the last pressure change. The pre-testand single breath data is then compared during the P_(crit) test (block138). A similar sequence is performed during a subsequent P_(opt) test.

In block 140, a determination is made whether the results of thecomparison in block 138 are significant. If significant and whereP_(crit) is being determined, P_(crit) is set to the current holdingpressure and a P_(crit) search complete flag is set (blocks 142, 144).The remote pressure is then set to the holding pressure prior to thechange in pressure (block 146), a pre-test flag is set TRUE to initiatea P_(opt) pre-test and a single breath test flag is set FALSE (block148).

During a P_(opt) search, P_(opt) is set to the remote pressure minus,for example, 1 cmH₂O (block 150). Further, a P_(opt) search completeflag is set TRUE, and the holding pressure is set to the new value ofP_(opt) (block 150).

Where the results of the comparison in block 140 were not significant, adetermination is made as to whether a flow alone condition exists (block152) or whether a flow only flag has been set TRUE (block 154). A flowalone condition is considered to exist when there is a significantchange in peak airflow but features of roundness and flatness have notchanged significantly. When a flow alone condition exists for twoconsecutive testing cycles during either a P_(crit) test or a P_(opt)test, P_(crit) or P_(opt) are set in blocks 144 and 150, respectively. Aflow only flag is set TRUE after a first flow alone condition so that asecond consecutive flow alone condition can be detected (block 154).Where a second flow alone condition has not yet occurred, pressure isreturned to the previous holding pressure, the pre-test flag is set TRUEand the flow only flag is set TRUE (blocks 156 and 158).

Where the comparison in blocks 138 and 140 was not significant, and aflow alone condition had not been established (block 152), a post-testflag is set TRUE to initiate a post-test (block 155) following a return(block 157). Further, a single breath complete flag is set FALSE (block155) so that the system will transition to the post-test at block 159.

Assuming the scan, pre-test, change pressure and single breath flags areall set FALSE, a post-test is initiated (block 159). During a post-test,a predetermined number of post-test breaths are collected (block 160).For example, five breaths are collected for a P_(crit) search, the lasttwo of which are averaged (blocks 162, 164). A determination is thenmade of whether this average is significant (block 166).

If the average is significant (i.e., significant flow limitation of thepharyngeal airway), P_(crit) is set to the remote pressure and aP_(crit) search complete flag is set TRUE (block 168). The pressure fromthe pressure generating means is then reset to the previous holdingpressure (block 169) so that the significant flow limitation is notmaintained. Further, a P_(opt) search is initiated by setting thepre-test flag TRUE (block 173). The pre-test and post-test flags arealso set FALSE (block 173) to permit subsequent re-initiation of apost-test during a P_(opt) search.

If the outcome of block 166 was a negative during the P_(crit) search,then a determination is made as to whether the P_(crit) scan wassignificant. If so then control flows to block 168 since the next 2cmH₂O pressure decrease would set P_(crit) to the significant pressurelimit of the scan test in the exemplary embodiment described herein(i.e., this example uses a 4 cmH₂O scan decrease and 2 cmH₂O P_(crit)search decreases). In this case, P_(crit) is set to the scan testpressure.

If the outcome of the block 167 was negative then in block 181 adetermination is made as to whether the pressure has been decreased tothe scan test pressure (i.e., previous holding pressure minus 4 cmH₂O).If the pressure has not yet been decreased to the scan test pressure,control flows to block 173 to initiate another P_(crit) search withoutexecuting a scan. However, if the pressure is at the scan test pressure,the scan test flag is set to TRUE (block 183), and the program flowprepares for another P_(crit) scan (e.g., with a holding pressure thatwill now be 8 cmH₂O below the original holding pressure of the initialscan).

Where the post-test breaths at block 160 were acquired during a P_(opt)search, a determination is made as to whether a prior increase inpressure during a P_(opt) search resulted in a large increase in flowleakage out of the mask after at least one prior P_(opt) test has beencompleted (block 174). If so, then the P_(opt) search complete flag isset TRUE (block 176) and pressure is returned to the previous holdingpressure at which a large increase in leakage was not detected (block170). Further, the scan test flag is set TRUE for the next series ofP_(crit) and P_(opt) searches, the pre-test flag is set FALSE and thepost-test flag is set FALSE (block 172).

If a large flow leak was not detected during a P_(opt) post-test (block174), the post-test breaths are used as pre-test data (block 178), and achange pressure request flag is set TRUE (block 180). This causesanother incremental increase in pressure and another P_(opt) test in aneffort to identify P_(opt).

Following a detection of P_(crit) and P_(opt) during a testing protocol,the system returns to a non-testing mode as indicated by block 78 inFIG. 7. A testing mode is then re-initiated in accordance with decisionblock 56 to adaptively update P_(crit) and P_(opt).

It will be appreciated by those of ordinary skill in the art that thepresent invention can be embodied in other specific forms withoutdeparting from the spirit or essential character thereof. The presentlydisclosed embodiments are therefore considered in all respects to beillustrative and not restrictive. The scope of the invention asindicated by the appended claims rather than the foregoing description,and all changes which come within the meaning and range of equivalentsthereof are intended to be embraced therein.

What is claimed is:
 1. Method for adaptively providing continuous positive airway pressure in an upper airway system comprising the steps of: detecting airflow in the upper airway system in predetermined increments of time; averaging said airflow information over a second period of time; distinguishing respiratory and non-respiratory components of said airflow using said averaged information; identifying periods of inspiration and expiration using said non-respiratory flow information; extracting features of said upper airway system during at least one of said periods of inspiration; identifying airflow profiles based on said detected airflow and said extracted features; determining a critical pressure which produces a first predetermined airflow profile; introducing incremental pressure increases to determine a second pressure, greater than said first critical pressure, which produces a second predetermined airflow profile; and setting pressure in said upper airway system using said second pressure.
 2. Method according to claim 1, wherein said step of determining further includes the steps of: establishing a predetermined airflow limit from at least said roundness feature, said flatness feature and a peak airflow signal, said predetermined airflow limit being adjusted in accordance with predetermined limit modifiers.
 3. Method according to claim 2, wherein said first and second predetermined airflow profiles correspond to peak airflow limits when said roundness and flatness features have not changed by more than a predetermined amount.
 4. Method according to claim 1, wherein said step of identifying test periods further includes the step of: initiating a test period when respiratory instabilities have been detected for a predetermined number of inspiration and expiration periods.
 5. Method according to claim 1, wherein said step of determining the non-respiratory component further includes the step of: detecting airflow leaks in an airflow path connected with said upper airway system.
 6. Method according to claim 1, wherein said step of determining a critical pressure further includes the steps of: reducing pressure in said upper airway system by a first predetermined amount; and incrementally reducing pressure in said upper airway system by a second predetermined amount less than said first predetermined amount.
 7. Method according to claim 1, further including the step of: storing extracted features and incremental pressure adjustments in memory.
 8. Method according to claim 1 further comprising the step of: incorporating an additional external input from an oxygen saturation monitor or a snoring monitor.
 9. Method according to claim 1 further comprising the step of: incorporating an additional external input from a monitor of pressure, sound, oxygen saturation or body position.
 10. Method for adaptively providing continuous positive airway pressure in an upper airway system comprising the steps of: detecting airflow in the upper airway system; averaging said airflow information over a predetermined period of time; determining respiratory and non-respiratory components of said airflow using said averaged information; identifying periods of inspiration and expiration using said non-respiratory flow information; and incrementally adjusting pressure in said upper airway system in response to airflow information detected during said period of inspiration.
 11. Method according to claim 10, wherein said step of adjusting further includes steps of: identifying a critical pressure at which a significant obstruction occurs during said inspiration; and identifying an optimum pressure for eliminating said obstruction during said inspiration.
 12. Method according to claim 11, wherein said step of identifying a critical pressure and an optimum pressure further includes the step of: introducing incremental pressure perturbations into said upper airway system; extracting characteristic features of said upper airway system during inspiration; using said extracted characteristics to identify the optimum pressure and the critical pressure.
 13. Method according to claim 10, wherein said step of adjusting is performed by identifying relative changes in airflow during said step of detecting.
 14. Method according to claim 10, wherein said step of adjusting further includes steps of: extracting features indicative of airflow resistance in said upper airway system.
 15. System for providing continuous positive airway pressure in an upper airway system comprising: means for detecting airflow in an upper airway system of a patient; means for generating pressure in said upper airway system in response to a command pressure; means for adaptively controlling said pressure generating means in response to said detecting means to automatically provide continuous positive airway pressure, said adaptive control means introducing incremental pressure perturbations for setting a command pressure relative to a predetermined critical pressure, means for averaging airflow information over a predetermined period of time and for determining non-respiratory flow using said averaged information; and means for identifying periods of inspiration and expiration using said non-respiratory flow information.
 16. System according to claim 15, wherein said adaptive control means identifies said critical pressure as a pressure at which a predetermined upper airway obstruction occurs during said inspiration.
 17. System according to claim 16, wherein said adaptive control means extracts features indicative of airflow resistance in said upper airway system to identify said critical pressure and to set said command pressure.
 18. System according to claim 16, wherein said extracted features include flatness, roundness and peak flow. 